German Patent Application No. DE 103 05 656 A1 describes such a method. According to this document, signals of a structure-borne noise sensor system coupled to the internal combustion engine are used as combustion features. German Patent Application No. DE 103 05 656 justifies this by stating that a simple relationship exists between a combustion noise emission and a fuel quantity of a pilot injection. To ascertain the injected fuel quantity, the signal of the structure-borne noise sensor system is detected and filtered in at least one first crankshaft angle range (measuring window) which is associated with the pilot injection, and in at least one second crankshaft angle range which is associated with a main injection.
Depending on the fuel quantities injected in the pilot injection and the main injection, a certain pattern of combustion features is obtained, from which a conclusion may be drawn on the injection points and injection quantities via comparison with reference patterns, so that the pilot injection may be corrected in a closed loop. This method is described in German Patent Application No. DE 103 05 656 using the example of an injection pattern composed of a pilot injection and a main injection. It should be applicable, however, to any desired combination of a first partial injection and at least one second partial injection, German Patent Application No. DE 103 05 656 mentioning pilot injections, main injections, and post-injections as partial injections elsewhere. Applicability of the method to two pilot injections is not mentioned in German Patent Application No. DE 103 05 656.
In addition to German Patent Application No. DE 103 05 656, a use of structure-borne noise sensors for the purpose of regulation in gasoline engines has been industrially used for years, for example, for knock control. In diesel engines, only systems having structure-borne noise regulation which corrects a single pilot injection per combustion are on the market.
To reduce the combustion noise, the implementability of two pilot injections per combustion is being increasingly required from vehicle manufacturers.
The pilot injection quantities are set via control times of electrically controllable injectors. Due to tolerances and aging (drift) of the injection system's components, an actual relationship between injected quantity and control time differs from a relationship that is stored, for example, in an injector characteristics map. A deterioration of the emissions (exhaust gas and noise) may result. This applies in particular to changes in the pilot injection quantities.
In addition to a drift of the injectors, a pressure wave in the injection system triggered by the injection affects the subsequent injections. In operation using a single pilot injection per combustion, these pressure waves may be corrected with the aid of fixed correction values ascertained on a test bench to the point that they have almost no negative effect on the main injection.
In systems having two pilot injections per combustion, however, considerable inaccuracies occur in the second pilot injection due to the pressure wave triggered by the first pilot injection. A correction of these inaccuracies using fixedly predefined correction values is not sufficiently accurate because the effect is a function of a plurality of influencing variables such as fuel temperature, fuel pressure, and fuel quality, which cannot be taken account using an acceptable degree of complexity.
A need therefore exists for an improved correction of the inaccuracies of pilot injections when at least two pilot injections are used. In tests using the method described in German Patent Application No. DE 103 05 656, it has been found that the effects of a plurality of consecutive pilot injections cannot be clearly separated in the structure-borne noise signal. This is explained by the fact that a plurality of pilot injections often combust in a very narrow angular range, so that their effect cannot be clearly separated in the cylinder pressure and in particular in the structure-borne noise.